1. Field of the Invention
The present invention relates to a method for producing a semiconductive single crystal. More particularly, the present invention relates to a method for producing a thin film of aluminum-doped ZnSe single crystal which is epitaxially grown on a ZnSe single crystal substrate by the MOCVD method.
2. Description of the Related Art
Presently, LEDs for emitting infrared, red, orange, yellow and green light are available. However, none of the practically used LEDs can emit blue light. To emit the blue light, a band gap should be larger than 2.5 eV.
Hitherto, many materials such as GaN, SiC and GaAlN have been proposed as materials of the blue light emitting LED. However, none of these materials has been able to provide a practically usable blue light emitting LED.
Since ZnSe has a band gap of 2.7 eV which is a direct transition type, it is one of promising materials of the blue light emitting LED. However, many problems should be solved before ZnSe is used as the material of the blue light emitting LED.
To produce LED, a p-n junction should be formed. To this end, a p-type thin film should be formed on a n-type substrate with low resistance, or a n-type thin film should be formed on a p-type substrate with low resistance. However, the ZnSe crystal as such is an n-type semiconductive material with high resistance. For example, the ZnSe crystal has resistivity of 10.sup.8 ohm.cm to 10.sup.9 ohm.cm. Therefore, the ZnSe crystal as such cannot be used as the substrate of LED.
Then, it is proposed to convert a bulk single crystal of ZnSe to the n-type one with low resistance through doping the ZnSe single crystal with impurities.
In addition, to form the p-n junction, the p-type film is also required. However, it was believed that any p-type ZnSe could be prepared.
Recently, the growth of the p-type ZnSe through Li-doping was reported by J. Nishizawa et al, "Blue Light Emission from ZnSe p-n Junctions", J. Appl. Phys., 57 (6), 2210-2216 (1985). But, the reported method has unsatisfactory reproducibility and the p-type ZnSe cannot be obtained easily.
The most fundamental problem resides in the production of a ZnSe bulk single crystal. Namely, a large ZnSe bulk single crystal with high purity has not been produced.
Since ZnSe is easily sublimated, it cannot be melted by simply heating it under atmospheric or moderate pressure. However, it can be melted under high pressure of about 80 atm. or higher. The melting point of ZnSe is reported to be about 1,520.degree. C., but this melting point is measured under such high pressure.
To produce the ZnSe single crystal, many methods such as a high pressure melting method, an iodine transporting method, a solution growth method, a sublimation method and the Piper method have been attempted. The first two methods can produce a comparatively large single crystal although the singly crystal contains a large amount of impurities and has many defects. The latter three methods produce a small single crystal, so that they are not practically applied. Further, the single crystal produced by these three methods has insufficient purity.
Since a large ZnSe single crystal with good quality cannot be produced by those conventional methods, many attempts have been made to epitaxially grow the n-type ZnSe thin layer on a GaAs substrate. Since the technique for growing the GaAs single crystal has been established, a large GaAs single crystal having less defects can be grown. Fortunately, difference of the lattice constants between GaAs and ZnSe is small.
The epitaxial growth of ZnSe/GaAs is disclosed by, for example, W. Stutivs, Appl. Phys. Lett., Vol. 38 (1981) 352 and K. Ohkawa et al, J. Appl. Phys. Vol. 62 (1987) 3216.
However, since the ZnSe/GaAs junction is a junction between the different materials, it has some drawbacks such as follows:
1. Although the lattice constants of both materials are close at room temperature, they have different coefficients of thermal expansion. Since the ZnSe film is grown on the GaAs substrate is at high temperature and cooled to room temperature, stress is generated in the thin film due to the difference of coefficients of thermal expansion between the two materials. Since the stress is surprisingly large, deterioration caused by stress leads to unsatisfactory electrical characteristics.
2. The impurities migrate from the substrate. For example, Ga atoms migrate from the substrate into the ZnSe thin film to form n-type impurities. Depending on the degree of migration, a carrier concentration varies.
These drawbacks are common to the heteroepitaxy.
To overcome these drawbacks, it may be contemplated to grow ZnSe on the ZnSe substrate. Hitherto, no ZnSe single crystal with good quality could have been obtained. Only one paper, namely P. Blaconnier et al, J. Appl. Phys. Vol. 52 (1981) 6895 reported the ZnSe/ZnSe epitaxy. Blaconnier et al grew the ZnSe thin film on the ZnSe substrate by the iodine transporting method or the Piper method. According to Blaconnier et al, the ZnSe thin film formed by the iodine transporting method contains many iodine atoms as impurities. In the photoluminescence measurement, strong emission due to excitons constrained with neutral donor impurities is observed. This is due to the presence of the iodine atoms. The contamination with the iodine atoms is an expected consequence since the iodine transporting method utilizes the following reaction: EQU ZnSe + I.sub.2 .revreaction. ZnI.sub.2 + 1/2Se.
The Piper method is one of the sublimation methods. The ZnSe thin film grown by this method has a strong emission line from excitons constrained with deep neutral acceptors. According to Blaconnier et al, the ZnSe thin film was formed by the MOCVD method using dimethylzinc [Zn(CH.sub.3).sub.2 ] and H.sub.2 Se as raw materials at the substrate temperature of 500.degree. C. The ZnSe thin film formed on the ZnSe substrate had a strong emission line from excitons constrained with the neutral donors. This can be attributed to the Ga atoms, which generated a deep donor impurity level. Blaconnier et al assumed that Ga might be contained in dimethylzinc. Although they did not intend to dope the impurities in the ZnSe single crystal, the Ga atoms were accidentally contained as the impurity.
In the prior arts, no report has been made on the epitaxial growth of a thin film of ZnSe doped with the n-type impurity on the ZnSe substrate.